Secrecy signaling system, method, and apparatus



Feb. 26, 1946. R. M. SPRAGUE SECRECY SIGNALING SYSTEM, METHOD, ANDAPPARATUS 3 Sheets-Sheet 1' Filed NOV. 19, 1943 ATTORNEY .Feb 26, E946.R. M. SPRAGUE SEQRECY SIGNALING SYSTM, METHOD, AND APPARATUS Filed Nov.19, 1943 3 Sheets-Sheet 2 INVENTOR BYMJ%AM' ATTORNEY R. M. sPRAGuESECRECY SIGNALING SYSTEM, METHOD, AND APPARATUS 3 Sheets-Sheet 3 FiledNDV; 19, 1943 M m R @f 0. m H m j www www r v mb R m l b mSwwQ E bx NMUMQNQ Y -2| Sx Q B \\\w wim wel im km www mw R 1 w l u v n u .,KQBQNQ mWQFQQQ +e Y Ev m Qwmmwwl Wsw Eva I Qm :@.WVL Vm@ :ww WT-- m -L ATTO R NEY Patented Feb. 26, 1946 TES UETED SECRECY SIGNALING SYSTEM, METHOD,AND APPARATUS tion of Delaware Application November 19, 1943, Serial No.510,943

19 Claims.

This invention relates to secrecy signaling systems and more especiallyto secrecy systems of the frequency scrambling type.

A principal object is to simplify and improve the secrecy system andapparatus disclosed in my prior application Serial No. 410,450, filedSept. 11, 1941.

Another object is to provide a frequency scrambling arrangement forsecrecy systems and the like, wherein the band width required fortransmitting the scrambled message is approximately the same as thefrequency band of the original signals,

Another object is to provide a simplified and improved secrecy systemwhich is useful over radio links as well as over wire lines.

Another object is to provide a system of secrecy transmission which iswell suited for commercial wire line communication systems.

A feature of the invention relates to a, modulating arrangement forfrequency scrambling systems whereby the number of possible scramblingcombinations is materially increased without increasing the band widthrequired for transmission.

Another feature relates to a secrecy system of the general typedisclosed in my prior application Serial No. 410,450, but wherein only asingle variable frequency modulator is required at the transmitter and acorresponding single Variable frequency modulator is required at thereceiver.

Another feature relates to the method of using haphazard recordedfrequencies to control a single variable frequency modulator whileachieving the scrambling power heretofore requiring two or more variableoscillators.

A further feature relates to a frequency scrambling system of the typeemploying haphazard frequency recordings as modulator controls, whereina substantial reduction is obtained in the band of recorded haphazardfrequencies.

A still further feature relates to the novel organization, arrangementand relative interconnection of parts which cooperate to produce asimplified and break-proof secrecy signaling system.

Other features and advantages not specifically enumerated will beapparent after a consideration of the following detailed descriptionsand the appended claims.

In the drawings which are illustrative of one preferred embodiment,

Fig. 1 is a schematic block diagram of a secrecy signaling systemaccording to the invention.

Fig. 2 is a chart showing the relation between original signal frequencyand the output or scrambled frequency for three arbitrary values of thehaphazard oscillator frequency,

Fig. 3 is a chart showing the relation between input and outputfrequencies for the various modulator stages, and also the relationbetween signal input frequency and output frequency for the over-al1modulator stages.

Fig. 4 is a schematic diagram of a recording blank arrangement fromwhich are derived the variable oscillator frequencies.

Fig. 5 is a modification of Fig. 4.

Referring to Fig. 1 the numeral l represents schematically anywell-known form of signal generator such for example as a microphone orother voice frequency source. The original voice frequencies are passedthrough a low-pass filter 2 to limit the frequency range to somepredetermined band e. g. 0-1800 cycles. This 0-1800 cycle band is thenpassed through another llter network 3 which is logarithmic betweeninput and output so that the fundamental and harmonics of the voicefrequencies are equalized in power and this equalization is spread overthe 0-1800 band. The signals from lter 3 are divided into two paths eachincluding a double-balanced modulator 4, 5 of the type disclosed inapplication Serial No. 501,928, filed Sept. 11, 1943. Modulator 4 issupplied with an 1800 cycle carrier from a suit-able source B, whilemodulator 5 is supplied with a 3600 cycle carrier from source l. Ifdesired, the 1800 and 3600 cycle carriers can be derived from the Samesource.

In the output of modulator l there appear substantially only the sum anddifference frequencies between the 1800 cycle carrier and the inputfrequencies from lter 3. Likewise in the output of modulator -ii'thereappear substantially only the sum and difference frequencies between the3600 cycle carrier and the input frequencies from filter 3. Modulator 4is followed by an 1800 cycle lowpass lter so that there appear at theoutput of filter 3 only the difference frequencies from modulator 4.Likewise modulator 5 is followed by a 3600 cycle low-pass'filter wherebythere appear at its output only the diiference frequencies frommodulator 5. The outputs of lters 8 and Sa-re then recombined thusgiving in the common output, Voice frequencies in two bands, theoriginal 0-1800 cycle band from filter 3 being now transposed to3600-1800 and 1800-0.

The transposed and recombined signal from the first modulator stage #Iis then fed into modulator stage #2 including a double balanced mod- 5.Modulator ulator l0 similar to modulators 4,

bear the same designation numeral. -ceiver the filter 3b is the converseof the filter 3 I is supplied with a carrier frequency from source IIwhich frequency is variable over the band 3800-5400 cycles but at ahaphazard rate. Consequently the dual voice frequency band 3600-1800 and1800- 0 from modulator #I is changed to the sum and differencefrequencies between the carrier from source II and the frequencies fromstage virl. The output of modulator I0 is followed by a 3600 cyclelow-pass filter I 2 which eliminates all the sum frequencies from At thereceiver the received scrambled fre-V quencies are passed throughsuccessive stages which are identical with the stages #L #2 and V#3. Inthe drawings the parts at the receiver which correspond to those at thetransmitter At the reat the transmitter so as to restore the originalpowerV relations in the fundamental and harmonies of the signalfrequencies, and of course the microphone is replaced by a suitablevoicefrequency recorder or reproducer Ib. It will be understoodthat thevariable frequencies from modulator `IIb at the receiver aresynchronized in any suitable manner with the frequencies from modulatorI I at the transmitter so that at any given instant each modulatorsupplies the identical frequency to its corresponding modulator I 0 or0b.

One preferred manner of deriving the variable 3800-5400 cycle carriersat the transmitter and receiver is schematically shown in Fig. 4. A pre-'viously made identical phonographic recording of the haphazardfrequencies on two phonograph blanks I1 and I'Ib is used respectively atthe transmitter and receiver.A These phonograph records are rotated byrespective motors I8 and I8b which are maintained in synchronism in anywell-known manner. Associated with each record is a pair of phonographpick-ups I9, 20 whose relative spacing is fixed, but whose jointposition with respect to the record I'I can be varied as desired. Bysuitable arrangements as described in my said prior application SerialNo. 410,450, the Y corresponding pick-ups I 9b and 200 at the receivercan be located at the proper orientation with respect to the record atthe receiver. The haphazard frequencies from the two pick-ups I9, 20 areadded by means of a double balanced modulator 2l similar to modulator 4and the modulater output is passed through a suitable highpass filterfor passing `the sum frequencies from modulator 2|. The same appliestothe pick-ups I9b, 2019.

In my prior application Serial No. 410,450, filed Sept. 11, 1941, twovariable frequency modulatorsrare required at the transmitter, and twosimilar synchronized variable frequency modulators are required at thereceiver. By using the method of `deriving the single variable frequencyabove described, the recording frequency of the serambling tracl ineach'phcnegrapir reccrd is reduced by approximately 50% and permitsdoubling the playing time of the records. The same number of scramblingcombinations are afforded as are afforded with the system of my priorapplication even though the phonograph records rotate at the same speedas the rotation of the records in the prior application. However, byrotating the records above -zdescribed at half the speed required in theprior application, the number of scrambling conditions is squared.

Fig. 2 shows in graph form the relation between the original signalvoice frequency and the output frequency from the filter I5 or filterI5b. The full-line graphs A show the relation when the oscillators lIIand IIb, are each delivering 3800 cycles. The dotted-line graphs B showthe relation at a Icarrier frequency of oscillators I I and IIb of 4600cycles; while the dot-dash line graph C shows the relation at afrequency of 5400 cycles.

'lrhegraphs of Fig. 3 show the individual input and output frequenciesfor the .three modulator stages at an instant when the oscillators IIand Hb are delivering for example 3800 cycles. In this figure, thefull-line graph D shows the relation between the input and outputfrequencies of modulator stage #L The dashed-line graph E shows therelation between the input and output frequencies of the modulator stage#2. The dotdash line graph F lshows the relation between the input andoutput frequencies -of ,modulator stage #3. The dotted line graphs Gshow the overall relation between the input frequency to stage #I andthe output frequency at stage #3. It will be noted that the graphs Gcorrespond tothe graphs A vof Fig. 2. A similar series of graphs fordifferent haphazard carrier frequencies of lthe oscillators H and H willshow that the output frequency of the third stage is confined to the 0to `1800 cycle band, notwithstanding the scrambling and successivemodulations to which the input signals have been subjected.

If there is any disturbance produced by an irregular motion of therecord Il, this difficulty is overcome in the following manner (Fig. 5).Assume that the frequencies recorded on the variable frequency trackrange between 1,900 and 2,700 cycles at some haphazard rate. Instead ofadding these frequencies directly the output from one vof the pick-uparms e. g., pick-up I9, is beatv in the modulator 25 against a fixedfrequency of 4,600 cycles from source 2.6, and the differency frequencytaken therefrom, When the pick-up arm I9 is scanning 1,900 cycles, theoutput from the modulator 25 delivers 2,700cyc1es (4600- 1900), and whenthe pick-up arm I9 is scanning 2700 cycles. the output from theauxiliary modulatoris 1,900 cycles. quencies is the same; The output ofthis auxiliary rnodulator 25 is then added to the output of the otherunaffected pick-up arm 20 and as before, the sum output frequencies `offrom 3,800

' to `5,400 cycles result. Now the variable frequency modulator isreceivingthe'same range .of frequencies as before .but the followinghas'been accomplished. When the driunorv turntable increases invfrequency dueto inaccurate gears, the output frequencies from each ofthe pick-up arms I9 and 20, increase proportionately. The outputfrequency of the unaffected arm 20 varies, of course, but the outputfrequency of. the! auxiliary mo V25 wif@creasesV when Yits inputvfluency increases, i Thus; the ,change Acf-requeney of the Thus, therange of fre- Y Vsum is now much less affected by changes in drum orturntable speed.

An example shows this improvement. Assume that each pick-up arm wasscanning 1,900 cycles and assume that the drum or record carrierrotation increased 10% in speed, then the output from each pick-upamplier would increase from 1,900 to 2,090 cycles per second, but theoutput from the auxiliary modulator 25 would decrease from 2,700 to2,510 cycles per second. Upon addition the sum frequency would be 4,600cycles (2510-2090l` as against Ll,600 cycles, had no speed increase beenpresent (1,900 plus 2,700). Thus, a change in drum speed has produced absolutely no change in sum frequencies. Consider the other extreme Whereone pick-up arm is scanning 1,900 cycles and the other pick-up arm isscanning 2,700 cycles. Under a 10% speed in crease, the unaiected 1,900cycle arm is delivering 2,090 cycles while the 2,700 cycles arm hasincreased to 2,970 cycles and output of the auxiliary modulator is 1,530cycles (4600-2970). Thus, the result is a sum frequency of 3,720. Nofrequency change due to gear inaccuracies would have delivered an outputfrequency of 3800 cycles. Thus, a 10% change in drum speed has producedonly an 80 cycle change in sum frequency, Thus, We see that this systemunder this extreme condition gives an improvement of 5.75 to l (460 to80), and under the other extreme condition, it gives an innitelyimproved condition. Therefore, we may assume that the averageimprovement is at least 10 to l and the gear requirements and tolerancesare therefore reduced by the same amount. it will be understood thatwhile a record medium in the form of a phonograph disc and a turntableare disclosed that any other well-known form of record medium andpick-up may be employed, for example a photographic sound film andassociated scanning equipment, or a magnetized wire or 1oand record.

It will be understood that in connection with the modification of Fig. 5that identical means are used at the receiving end as shown.

While certain particular embodiments of the invention have beendescribed, it will be understood that various changes and modificationsmay be made therein Without departing from the spirit and scope of theinvention.

What is claimed is:

l. rEhe method of secret signal transmission which comprises,transposing the original frequency succession into a different frequencysuccession occupying a band width greater than that of the originalfrequencies, generating a haphazardly variable frequency carrier,modulating said carrier by said other frequencies, selecting one sideband of the modulations, and remodulating each side band to restore theoriginal band width but without restoring the original frequencysuccession.

2. The method o-f secret signal transmission which comprises,transposing the signal frequencies into other frequencies occupying aband approximately twice the width of the original band, modulating saidtransposed band on a haphazardly variable carrier whose frequency ishigher than the width of the transposed band, the frequency excursion ofsaid variable carrier being less` than the band Width of the originalfrequencies, selecting one side band from the modulated variablecarrier, remodulating the selected side band on another carrier of lowerfrequency than said variable carrier to restore the original band widthwithout restoring the original frequency successions. A

3. The method of secret signal transmission which comprises, conning theoriginal signals to a band e. g., ,fo-fi, modulating said hand on acarrier of frequency f1, simultaneously modulating said band on anothercarrier of frequency nfl, selecting only the difference frequencies fromeach modulated carriery combining said difference frequencies,modulating a haphazardly varying frequency carrier by said combineddifference frequencies, selecting from said modulated Variable frequencycarrier the difference frequencies, modulating the lastmentionedselected frequencies on a carrier of nfl, and selecting from thelast-mentioned modulated carrier the difference frequencies. Y Y

4. The method according to claim 3 in which said variable frequencycarrier is varied at a haphazard rate but over a frequency band whosewidth is less than ,fof-f1.

5. The method of secret signal transmission of an original signalfrequency spectrum of a predetermined band width which comprises,subjecting the signals to successive carrier modulations, one of whichis of thehaphazardly variable carrier type whereby the nal signals havea frequency spectrum of approximately the same width as the originalsignals but with at least a portion of the. original frequency spectrumscrambled.

6. The method of secret signal transmission of an original signalfrequency spectrum of a predetermined band width which comprises,equalizing the power in all frequencies of the signal band, subjectingthe signal frequencies to at least three successive carrier modulationstages, the first stage producing an inverted frequency spectrum theband width of which is a multiple of the original spectrum, the secondstage producing a scrambled frequency spectrum, and the third stageproducing a scrambled frequency spectrum of substantially the same widthas the original band.

7. The method of secret signal transmission of an original signalfrequency spectrum which comprises, subjecting said spectrum at atransmitter to successive carrier modulations including modulation of ahaphazardly variable carrier, one of which is a varying frequencycarrier stage to produce a frequency spectrum for transmission ofsubstantially the same band width as the original signals but with thefrequencies scrambled, receiving the last-mentioned band and subjectingit to successive carrier modulations similar to those at the transmitterwith the varying carrier at the receiver synchronized with the varyingcarrier at the transmitter to reproduce the original frequency spectrum.

8. In a secret signaling system, a source of signal frequency spectrumhaving a predetermined band width, means to subject said spectrum tosuccessive frequency changing stages, one of which is of the haphazardlyVarying carrier type for scrambling the frequencies, and means to selectfrom the last modulation stage a frequency spectrum of substantially thesame band width as the original spectrum but with the frequenciesscrambled.

9. In a secret signaling system, a source of signal frequencies Within apredetermined spectrum, three successive frequency-changing stages forsaid spectrum, the first stage having means to expand the spectrum to amultiple of the original, the second stage having means to scramble thesignals from the first stage in a haphazard manner, and the third stage`having means lto restore the band width ofthe scrambled frequencies tothat of the original band width.

y10.. A secret signaling system according Yto claim 9 in which the.original spectrum has an upper .frequency of il and the said firststage ,comprises a pair oi modulators one of which is supplied with acarrier of approximately f1 and the other of which is supplied with acarrier approximately nfi, means to select from the .ii-rst modulatorthe spectrum below f1, means to select from the second modulator the.spectrum below nfl, means to combine the two .selected spectrums; thesaid second .stage comprises a modulator upon which said combinedspectrums are impressed and a source of haphazardly varying carrierconnected thereto, said carrier having a band width which is less thanthat of the original spectrum but with its lower frequency limit ,abovenfl, and means to select from said second modulator stage the spectrumwith a maximum frequency limit of approximately nii; and said thirdAstage comprises a modulator supplied with carrier .of fre- `quency nii,and means to select from its output a spectrum of substantially the samewidth as the original spectrum but with the frequencies scrambled. Y

11. A secret signaling system according to claim 9 in which the said rststage comprises, a pair of double-balanced modulators upon which theoriginal signals are separatelyl impressed and means to select andcombine from the said pair of double-balanced modulators the differencefrequencies; said second stage comprising another double-balancedmodulator upon which the combined difference frequencies from the rststage are impressed, the double-balanced modulator oi the second stagebeing connected to a source ci carrier of haphazardly varying frequencyand means to select from the output of said variable carrier modulator aspectrum which Yis approximately the same band Width as the input handwidth; said third stage comprising adouble-balanced modulator upon whichthe selected spectrum from the second stage is impressed and means toselect from the output of said third stage a spectrum of substantiallythe same width as the original spectrum but with the frequenciesscrambled.v

12. A system according to claim 9 in which the selected spectrum fromthe third stage is fre- .ceived by a receiver having substantially thesame frequency-changing stages as at the transmitter.

13. vA secret signaling system comprising a source .of signals of .apredetermined limited irequency spectrum, and a Vplurality offrequencychanging stages for scrambling the `original frequencies Vwhilemaintaining the original band width', the last-mentioned means includingacarrier modulation stage which is Supplied with a haphazardly varyingcarrier.

14. .A secret signaling system according to claim 13 in which-the sourceof haphazardly varying carrier comprises a record of frequencies havinga frequency succession which is haphazard, a pair of pick-upscooperating with said record at different portions thereof, and means tocombine the signals from said picklps to produce a single signal ofvariable frequency.

15. A secret signaling system according to claim i3 in which the sourceof variable carrier frequency comprises a record of frequencysuccessions varying haphazardly, a plurality of pick-ups associated withdifferent portions of said record to produce simultaneously differentfrequencies, a double-balanced modulator upon which the `signals fromsaid pick-ups are impressed, and means to select one side band of theoutput of said double-balanced modulator.

16. Apparatus for producing a haphazardly varying frequency ior secretsignaling systems comprising a record medium embodying a record ofhaphazardly succeeding frequencies, a plurality vof pick-ups associatedwith diierent portions .of said record to produce simultaneouslydiierent frequencies under control of said record, and means to combinethe signals from said pick-ups to produce a single signal whosefrequency varies haphazardly 'but as a function of the records of vtherecorded frequencies.

17. Apparatus according to claim 16 in which a frequency inversionmodulator is provided between one pick-up and the said combining meansto compensate for irregularities in the movement of the record.

18. Apparatus according to claim 16 in which a dilerence frequencymodulator is provided between one pick-up device and said combiningmeans whereby irregularities in the movement of the record carrier arecompensated for.

19. A re-entrant band shifter for an original frequency band fil-f3,comprising means to shift the band to a different spectrum fz--fi wheref2 is intermediate fi--fs and wherein the band fz-fi is wider than theband fia-f3, means to restore the portion of the band f2-f4 which doesnot overlap the band f1f3 to 'the non-overlapping portion of the bandfl-fs but with the frequency succession of the said restored portiondifferent from the corresponding original portion of the band f1--3, andthe resultant spectrum is of the same width as fi-fa but with adifferentrfrequency succession from the said original frequency band.

ROBERT M. SPRAGUE.

